Saturday, August 3, 2013

Out of Wood

St. Paul, MN (Jul 31, 2013)

St. Paul in Minnesota has a power plant smack at the center of the city. You would think they are crazy, right? Well no, the plant is actually there to reduce, rather than increase, urban pollution, saving energy in the process. The plant is powered by biomass. Basically organic trash, mostly wood chips obtained by disposing dead trees and branches. This is carbon that would be released anyway as CO2 into the atmosphere as the wood decomposes, so burning it is actually carbon-neutral. The best part of the story, however, is that this is a cogeneration plant. A typical power plant wastes up to 60% of the energy produced as waste heat. Cogeneration plants use the waste energy to heat the buildings nearby (district heating), pushing in this way the efficiency above 80%. That saves a lot of energy in winter (it is cold in Minnesota), and cleans the air because one central plant is much more efficient than many smaller burners for each individual building.

Gotcha!

But it gets even better: power plants need to be kept running all the time to maintain their peak efficiency, even though at night the need for energy is lower. The St. Paul plant uses this excess energy to chill large amount of water, that is then used to cool in summer the same buildings that are heated in winter (district cooling). All this is not really new: a cogeneration (gas powered) district heating plant has been operating in my hometown for at least 30 years, heating the houses of half million people (half of the total city population). My university also has a cogeneration plant since 1891 (yes XIX century, that's not a typo) that heats and cools the whole campus (and provides it with electricity). But for some reasons I never thought to take a picture of the ugly cogeneration gas plant in Torino, or the even uglier coal cogeneration plant at Iowa State, so here it is, the one in St. Paul. Plus it gives me an excuse to show the St. Paul panorama at the bottom (taken from the Guthrie Theatre terrace), and the photo on the left, showing my sister and one of my nieces. The photo is in punishment because she (my little niece Letizia) is making a point in hiding when I try to take a picture of her, but I was sneaky and fast, and I got her just fine!